The present invention relates to a gas phase process for the preparation of aromatic isocyanates by reaction of corresponding primary amines with phosgene. In this process, phosgene and one or more primary aromatic amines are reacted above the boiling temperature of the amine(s) in a reactor having a reaction space which is essentially rotationally symmetric to the direction of flow. The flow rate, averaged over the cross-section, of the reaction mixture along the axis of the essentially rotationally symmetric reaction space in the section of the reaction space in which the conversion of the amine groups into isocyanate groups is between 4 and 80% is not more than 8 m/sec. The flow rate, averaged over the cross-section, of the reaction mixture along the axis of the essentially rotationally symmetric reaction space in this section of the reaction space is always below the flow rate averaged over the cross-section at the start of this section.
Isocyanates are prepared in large amounts and serve chiefly as starting materials for the preparation of polyurethanes. They are usually prepared by reaction of the corresponding amine(s) with phosgene. One possible method for preparing isocyanates is reaction of the amine(s) with the phosgene in the gas phase. This process, which is conventionally called gas phase phosgenation, is distinguished in that the reaction conditions are chosen such that at least the reaction components (i.e., amine, isocyanate and phosgene), but preferably all the educts, products and reaction intermediate products, are gaseous under the conditions chosen. Advantages of gas phase phosgenation include a reduced phosgene hold-up, the avoidance of intermediate products which are difficult to phosgenate and increased reaction yields. The present invention relates exclusively to gas phase phosgenation.
Various processes for the preparation of isocyanates by reaction of one or more amines with phosgene in the gas phase are known from the prior art.
EP-A-289 840 describes preparation of diisocyanates by gas phase phosgenation in which a turbulent flow at temperatures of between 200° C. and 600° C. in a cylindrical space without moving parts is employed. By dispensing with moving parts, the risk of discharge of phosgene is reduced. According to the teaching of EP-A-289 840, for it to be possible to carry out the process disclosed in EP-A-289 840, it is essential for the dimensions of the tube, reactor and the flow rates in the reaction space to be such that a turbulent flow which is characterized by a Reynolds number of at least 2,500, preferably at least 4,700 prevails in the reaction space. According to the teaching of EP-A-289 840, this turbulence is in general ensured if the gaseous reaction partners pass through the reaction space with a flow rate of more than 90 m/s. Due to the turbulent flow in the cylindrical space (tube), disregarding fluid elements close to the wall, a relatively good flow equipartition in the tube and therefore a relatively narrow dwell time distribution is achieved, which, as described in EP-A-570 799, leads to a reduction in the formation of solids. A disadvantage of the process disclosed in EP-A-289 840 is that because of the necessary high flow rates, realization of the dwell time necessary for complete reaction of the amines, especially if aromatic amines are employed, is possible only in very long mixing and reactor tubes.
EP-A-570 799 discloses a process for the preparation of aromatic diisocyanates in which the reaction of the associated diamine with the phosgene is carried out in a tube reactor above the boiling temperature of the diamine within an average contact time of from 0.5 to 5 seconds. As described in the specification, reaction times which are too long and also those which are too short lead to an undesirable formation of solids. A process is therefore disclosed in which the average deviation from the average contact time is less than 6%. Maintenance of this contact time is achieved by carrying out the reaction in a tubular flow which is characterized either by a Reynolds number of above 4,000 or by a Bodenstein number of above 100. If the tubular flow is characterized by a Reynolds number above 4000, it is also a disadvantage because high flow rates make realization of the dwell time necessary for complete reaction of the amines possible only in very long mixing and reactor tubes. According to the teaching of EP-A-570 799, the approximately ideal plug flow characterized by a Bodenstein number of at least 100 can also be realized by use of installed elements in the reaction tube which counteract the formation of a laminar flow profile and have the effect of the formation of an even flow front, installed elements in the form of a three-dimensional fine-mesh wire grid or packing bodies are disclosed. A disadvantage of these process variants is that due to the installed elements, the risk of formation of deposits in the reaction tube is increased. These deposits can lead to blockages and/or inhomogeneities in the approximately ideal plug flow sought.
EP-A-699 657 describes a process for the preparation of aromatic diisocyanates in the gas phase in which the reaction of the associated diamine with the phosgene takes place in a reactor made up of two zones. The first zone, which makes up about 20% to 80% of the total reactor volume, is mixed ideally. The second zone, which makes up 80% to 20% of the total reactor volume, can be characterized by a piston flow. The second reaction zone is preferably designed as a tube reactor. However, because at least 20% of the reaction volume is back-mixed in an ideal manner, a non-uniform dwell time distribution results, which can lead to ah undesirable increased formation of solids.
Optimization of the use of tube reactors for gas phase phosgenation (as disclosed, e.g., in EP-A-570 799) using the jet mixer principle (Chemie-Ing.-Techn. 44 (1972) p. 1055, FIG. 10) is the subject matter of numerous applications.
EP-A-1 362,847 teaches that equalizing the educt stream fed via the annular space of the tube reactor and feeding of the two educt streams as centrally as possible into the tube reactor has a great positive influence on the stability of the reaction zone and therefore on the gas phase reaction overall. As a consequence of the more stable reaction procedure, the observed temperature variations decrease significantly and the asymmetry in the temperature distribution observed without the measures disclosed therein disappears almost completely. EP-A-1 362 847 also teaches that temperature variations and asymmetries in the temperature distribution lead to the formation of by-products, which lead to caking and blockages in the reactor and therefore to a shortening of the service life of the reactors. However, specific indications for conversion of the process disclosed into an industrial scale are not disclosed in EP-A-1 362 847.
As described in EP-A-1 555 258, if the tube reactors employed are increased in size, an increase in the size of the mixing nozzle, which is often constructed as a smooth jet nozzle, is also necessary. As the diameter of the smooth jet nozzle increases in size, however, the speed of mixing of the central jet is also reduced due to the longer diffusion path required, and the risk of back-mixing is increased, which in turn leads to the formation of polymeric impurities and therefore solid caking in the reactor. According to the teaching of EP-A-1 555 258, the disadvantages described can be eliminated if the one educt stream is injected at a high speed via an annular gap located concentrically in the stream of the other educt. As a result, the diffusion path for the mixing is small and the mixing times are very short. The reaction can then proceed with a high selectivity to the desired isocyanate. The formation of polymeric impurities and the development of caking are thereby reduced. EP-A-1 55 258 also teaches that use of comparable speeds of the components at the mixing point has the result that significantly shorter reaction spaces are required to achieve the maximum temperature in the reaction system than when conventional smooth jet nozzles are employed.
According to EP-A-1 526129, an increase in the turbulence of the educt stream in the central nozzle has a positive influence on the mixing of the reactants and therefore on the gas phase reaction overall. As a consequence of the better mixing, the tendency towards the formation of by-products decreases and the dwell time required and therefore reactor construction lengths drop significantly. EP-A-1 526129 teaches that the mixing zone may be shortened to 42% of the original length if a spiral coil is employed as a turbulence-generating installed element in the central nozzle.
EP-A-1 449 826 discloses a process for the preparation of diisocyanates by phosgenation of the corresponding diamines, in which the vaporous diamines, optionally diluted with an inert gas or with the vapors of an inert solvent, and phosgene are heated separately to temperatures of from 200° C. to 600° C. and are mixed and reacted in a tube reactor. In this process, a number n≧2 of nozzles aligned parallel to the axis of the tube reactor are arranged in the tube reactor, the stream containing the diamines is fed to the tube reactor via the n nozzles and the phosgene stream is fed to the tube reactor via the remaining free space. According to EP-A-1 449 826, advantages of this process include a shortening of the mixing times compared with a single nozzle (individual nozzle) with the same cross-sectional area and associated with this a shortening of the dwell time required in the reactor (investment costs advantage).
A further development of the use of tube reactors for gas phase phosgenation such as has been disclosed in EP-A-570 799 using the jet mixer principle (Chemie-Ing.-Techn. 44 (1972) p. 1055, FIG. 10) is the subject matter of WO2007/028715. WO2007/028715 discloses a process for the preparation of isocyanates by phosgenation of the corresponding amines in the gas phase in a reactor having a mixing device and a reaction space. According to WO2007/028715, the reaction space includes in the front region the mixing space in which mixing of the gaseous educts phosgene and amine, optionally mixed with an inert medium, predominantly takes place which mixing, as a rule, is accompanied by the start of the reaction. According to WO2007/028715, essentially only the reaction and at most to a minor extent the mixing then takes place in the rear region of the reaction space. Preferably, in the process disclosed in WO2007/028715, reaction spaces which are rotationally symmetric to the direction of flow and which can be broken down in construction terms essentially into up to four longitudinal sections along the longitudinal axis of the reactor in the course of flow are employed. The longitudinal sections differ in the size of the flowed-through cross-sectional area. A disadvantage of the process disclosed is the high flow rate of from 10 to 300 m/s, preferably from 40 to 230, more preferably from 50 to 200, even more preferably, from more than 150 up to 190 and most preferably from 160 to 180 m/s, with which the gaseous reaction mixture passes through the reaction space. As already described in EP-A-570 799, because of the high flow rates, realization of the dwell time necessary for complete reaction of the amines, especially if aromatic primary amines are employed, is possible only in very long reactor tubes. Another disadvantage is that the changes in the flowed-through cross-sectional area of the reaction space are generated by a volume body in a tube reactor, and the conversion of the reactor construction disclosed into an industrial scale is therefore expensive in construction terms. It is furthermore a disadvantage of volume bodies in tube reactors that, like the installed elements according to the teaching of EP-A-570 799, these increase the risk of the formation of deposits in the reaction tube, which lead to blockages and therefore to a shortened service life of the reactors.
WO2008/055898 discloses a process for the preparation of isocyanates by phosgenation of the corresponding amines in the gas phase in a reactor, in which (analogously to WO2007/028715) the reactor employed has a mixing device and a reaction space, and the rotationally symmetric reaction space can be broken down in construction terms into up to four longitudinal sections along the longitudinal axis of the reactor in the course of the flow, the longitudinal sections differing in the size of the flowed-through cross-sectional area. Compared with WO2007/028715, however, the changes in the flowed-through cross-sectional areas are achieved not by a volume body installed in a tube reactor but by a corresponding widening or constriction of the outer wall of the reactor. A disadvantage of the disclosed process is the high flow rate of from 10 to 300 m/s, preferably from 40 to 230, more preferably from 50 to 200, even more preferably from more than 150 to 190 and most preferably from 160 to 180 m/s, with which the gaseous reaction mixture passes through the reaction space, according to the teaching of WO2008/055898 in sections of the same or increasing area with area being chosen so that the average speed of the reaction mixture is in general greater than 60 m/s. The process disclosed in WO2008/055 898 indeed avoids volume adjusting components in the reaction space and therefore reduces the risk of the formation of deposits compared with the process disclosed in WO2007/028 715, but the disadvantage of the high flow rate remains. Because of the high flow rates, realization of the dwell time necessary for complete reaction of the amines, especially if aromatic primary amines are employed, is possible only in very long reaction spaces.
EP-A-1 275 639 also discloses a process for the preparation of isocyanates by phosgenation of the corresponding amines with phosgene in the gas phase using a reactor in which the reaction space has a widening of the flowed-through cross-sectional area in the direction of flow after the mixing of the two educts. According to the teaching of EP-A-1 275 639, this widening of the flowed-through cross-sectional area can be sudden, and the reaction space of the reactors employed in the process disclosed can also have a cascade-like and/or continuous change in the flowed-through cross-sectional area. According to the teaching of EP-A-1 275 639, if a cascade-like and/or continuous change is employed, the course of the speed of the reaction mixture along the axis of the reactor can be adjusted. According to EP-A-1 275 639, a narrowing of the cross-section or preferably a slight widening up to twice, preferably up to 1.5 times the starting cross-section leads to an acceleration of the flow during the reaction because of the increase in volume, which stabilizes the flow and counteracts the risk of back-flows. By a suitably chosen widening of the cross-sectional area, the flow fate of the reaction mixture can be kept just constant over the length of the reactor. As a result, the reaction time available increases for a constant length of the reactor.